A hydraulic or electric motor is used in a conventional injection mechanism for moving a screw in an axial direction to inject a molten molding material. For this reason, the horsepower and size of a motor must be increased in order to perform high-pressure, high-speed injection. However, when the motor size is increased, the injection molding apparatus becomes expensive as a whole. Furthermore, the rotor size of the motor must be increased to prolong the response time due to inertia influence, resulting in inconvenience.